1. Field of the Invention
The present invention relates to a lamp and more particularly to a single ended subminiature fluorescent lamp in which power supply terminals are preferably arranged on only one end of a bulb and wherein the bulb has the same shape as a typical halogen lamp used as a signal light.
2. Description of the Related Art
FIG. 5 is a cross sectional view taken along lamp axis Z of a related art fluorescent lamp 90. FIG. 6 is another cross sectional view of the related art lamp taken along line B--B of FIG. 5. The related art fluorescent lamp 90 includes: a bulb 94; fluorescent material 94a coated on an internal surface of the bulb 94; a stem 91; a discharge chamber 94b filled with gas and mercury; lead wires 91a, 91a', 91a" passing air-tightly through the stem 91; a thermal cathode filament 92 supported by the lead wires 91a and 91a'; an electron emitting material 92a coated on the thermal cathode filament 92; and a ring-like anode 93 supported by the lead wire 91a". At startup of the related art fluorescent lamp 90, a DC voltage of 5 V is applied between the lead wire 91a and the lead wire 91a' and thermal electrons are emitted. Then, a DC voltage of 24V is applied between the thermal cathode filament 92 and the ring-like anode 93, causing the thermal electrons emitted from the thermal cathode filament 92 to be directed to the ring-like anode 93 such that discharge starts, the fluorescent material 94a is excited, and light is emitted. The related art fluorescent lamp 90 has several drawbacks and problems. First, converting efficiency from wattage to luminance of the related art fluorescent lamp 90 is approximately 3.7 lm/W. This converting efficiency results in a light quantity that is enough to use as a signal light, but is not enough for use as a back-light for a liquid crystal display.
Second, as shown in FIG. 7, discharge should normally occur between the ring-like anode 93 and a grounded end S of the thermal cathode filament 92. However, in the related art florescent lamp 90, discharge spot P on the grounded end S moves towards end U of the positive side of the thermal cathode as lighting time passes due to deterioration of the electron emitting material 92a. When a DC voltage of 5 V is applied to the thermal cathode filament 92, the discharge spot P on the thermal cathode filament 92 moves to center point T of the thermal cathode filament 92. Center point T has a voltage that is approximately 2.5V higher than the voltage at the grounded cathode end S. In other words, electric potential between the thermal cathode filament 92 and the ring-like anode 93 decreases approximately 2.5 V. Thus, the discharge current is decreased and luminance of the subminiature fluorescent lamp 90 is also decreased. Furthermore, when the discharge spot P passes the center point T of the thermal cathode filament 92 and further moves toward the positive side of the end U of the thermal cathode, the luminance of the fluorescent lamp 90 greatly decreases as shown in line BO of FIG. 3. This decrease in luminance is a result of the discharge distance between the discharge spot P and the ring-like anode 93 increasing in addition to voltage decreasing between the thermal cathode filament 92 and the ring-like anode 93.
Third, the anode 93 has a hollow interior, and the interior space is filled with a getter material and mercury alloy. Therefore, design of the fluorescent lamp 1 is limited by dimensional requirements. Moreover, it is impossible to decrease the external diameter of the fluorescent lamp to less than 4 mm.